1. Field of the Invention
This invention relates to a method and an apparatus for providing non-conductive transparent substrates with transparent coatings having optical properties of good uniformity, and particularly relates to a glass rinsing operation that avoids sources of non-uniformity of said optical properties.
2. Description of the Prior Art
In the past, transparent metal coated glass articles have been produced by various vapor deposition techniques which generally involve the deposition, from the vapor phase, of substantially pure metals, such as nickel or chromium on a prepared glass substrate. However, while such techniques are generally capable of providing metal films of acceptably uniform thickness and specified visual transparency, commercial films of this type have been observed to possess an undesirably high number of visible pin-holes. Further, this process is expensive and complex. Metal coated metallic and non-metallic articles have been produced by various well-known electroless or chemical plating techniques. These techniques generally involve the immersion of a metallic article or a sensitized non-metallic article into a suitable electroless plating bath comprising an aqueous medium having dissolved therein a metal salt and an appropriate reducing agent, whereupon a metal film is deposited upon the immersed article by an autocatalytic mechanism.
The electroless process is an old and established one. For example, Brenner and Riddell disclosed in 1944 that an opaque coating of nickel could be autocatalytically deposited upon metallic substrates by immersing the substrates into a nickel salt solution containing sodium hypophosphite. U.S. Pat. Nos. 2,532,283 and 2,532,284 were issued to Brenner and Riddell upon their discoveries. The use of sodium hypophosphite as the reducing agent results in deposits which are not pure metal, but which contain about 2 to 10 percent elemental phosphorus by weight. In this connection, it is known that the presence of phosphorous in a deposited nickel film affects certain of the film characteristics, including its dominant wave lengths, infrared absorption characteristics, excitation purity and electroconductivity. In addition, and for reasons not wholly understood, it has been found that the uniformity of deposited nickel-phosphorous films generally decreases rapidly with increased thickness when the thickness of the coated glass substrates is greater than about three-sixteenth of an inch.
Other electroless immersion plating processes involve the use of boron-containing reducing agents which are effective at room temperature. U.S. Pat. Nos. 2,968,578, 3,140,188, 3,096,182 and 3,045,334 are representative of improved electroless plating processes of this type. U.S. Pat. No. 2,956,600, issued to Carlson et al., describes a spraying process wherein two separate solutions are sprayed upon substrates to form nickel coatings. This process uses sodium hydrosulfite and sodium hypophosphite as a reducing agent.
To a large extent the prior art has been concerned with production of opaque coatings by electroless coating, although it is understood that the assignee of U.S. Pat. No. 2,702,253 produces a glass plate having a transparent nickel coating possibly by the process therein disclosed. The problem of producing transparent glass or like articles is much more difficult because relatively minute variations in thickness are readily visible to the naked eye as unsightly defects. Other variations in such coatings can provide streaks with a glass region appearing almost opaque due to reflection of light in an otherwise transparent glass plate.
Many solutions suggested by the prior art develop a coating of gradually increasing thickness well beyond thicknesses which are opaque. The production of uniform transparent films with such solutions is especially difficult.
U.S. application Ser. No. 57,451, filed on July 23, 1970, in the name of Richard G. Miller, teaches a method whereby transparent substrates such as glass are provided with a uniform transparent coating by contacting the glass simultaneously with a mixture of a reducible metal salt in a solution and a reducing agent, which mixture becomes rapidly depleted of its film forming capacity before the resulting coating becomes opaque. This produces a uniform coating at a rate which is relatively rapid and then relatively slower and which effectively ceases to produce coating while the coating remains transparent. According to said application, it has been found that by using such mixtures and discontinuing the contact therewith after the rate of deposition of coating has reached the slower rate, transparent films of improved uniformity with few pin-holes can be achieved. The application further states that coatings of the best uniformity may be obtained even with large plates having four or more square feet of surface when the coating is applied by separately spraying a solution of reducing agent and a solution of the reducible metal salt on the glass plate preferably while the major surfaces thereof are in a horizontal or substantially horizontal plane. The process has been found to be effective over a broad temperature range for coating any of the so-called catalytic metal substrates or non-catalytic substrates sensitized in a conventional manner to promote deposition of continuous, adherent transparent metal films. An advantage of the process is that it will deposit highly uniform transparent films when performed at about room temperature, i.e., from about 20.degree. Centigrade to about 30.degree. Centigrade. The application further states that in order to insure that each of a plurality of substrates is provided with a coating that exhibits substantially the same physical and chemical characteristics, it is advantageous that the process temperature be held constant to within about .+-. 1.degree. Centigrade, for example, over 100 substrates or over 1000 square feet of substrates, or the like. Best uniformity and appearance of transparent films is achieved when films are deposited to a thickness having a luminous transmission of about 35 to 40 percent or less, and when the films comprise nickel-boron, cobalt-boron, iron-boron, and the like. Films comprising mixtures of boron and nickel, cobalt and/or iron may also be provided. In all such films, the boron is present in a minor amount (rarely exceeding about 15 percent by weight and normally between about 2 and 7 percent by weight) while the metal (nickel, cobalt and/or iron) is present in preponderant amounts (rarely less than 85 percent by weight and normally between about 93 and 98 percent by weight).
The transparent substrates obtained in accordance with the last-mentioned process may be employed, for example, as transparent windows or outside walls in a building such as a skyscraper or other multistory structure. These substrates may be especially advantageously employed as one of the plates which make up multiglazed units as described in the previously mentioned U.S. patent application Ser. No. 57,575. It will be understood that uniformity of coating in such uses is especially important because otherwise the reflected color portions of the building differs sharply from that of other portions, thus distracting from its appearance.
To generalize, the prior art teaches a process whereby glass plates are conveyed along an article movement path through a sequence of rinse and spray stations. After a plate leaves a rinse station, it is normally covered with a layer of water. When the plate enters a spray station, this layer of water is normally pushed toward the trailing edge of the plate and tends to accumulate toward the trailing edge. As more water accumulates at the trailing edge, the water tends to flow back into the spray zone. The problem is especially severe at the trailing edge where water is retained by the edge of the plate. After reaching equilibrium thickness at the trailing edge, some of the water flows back into the spraying zone so that the glass plate is sprayed while it is covered with one or more puddles of water of non-uniform thickness. As a result, there is an uneven dilution of the sprayed-on solutions in the sensitizing and coating stations, and this causes the coating on the plate to vary in thickness, in absorption and in reflectance. This is extremely pronounced adjacent to the trailing edge of the plate. In addition, a finger-like front of the sprayed-on solution forms between each spray station and each rinse station which manifests itself in streaks in the direction of travel of the plate. In addition, splashing in the sensitized area causes streaks and mottle in the coating. The prior art recognized the existance of the problem of non-uniformity of coating, but failed to associate its relation to an inferior rinsing technique.
In U.S. patent application Ser. No. 159,747, filed on July 6, 1971, in the name of the present inventor now U.S. Pat. No. 3,761,305, there is disclosed a solution to this problem. In the said mentioned application, there is suggested the use of a squeegee/shield combination ahead of the spray station to insure that there are no puddles or finger-like fronts of rinse water at a spray station. The squeegee is made of soft rubber or plastic, and it prevents the formation of finger-like fronts and also protects the glass surface from splashing and defects associated with splashing. While this is an advance over the prior art, the squeegee is not an ideal solution to the problem in that there is always a chance of damaging the sensitization and activation layers when solid particles that may become trapped between the glass and the squeegee or in the squeegee material itself abrade said layers.